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They may know why St. Clair Highway is sinking, but city officials don’t know yet how to fix it.

A 200-foot steel wall was installed five years ago along the Pine River to stop the ground movement discovered months after the street was reconstructed. Two years after that, the highway between Palmer Road and Oak Street was closed.

It’s been that way ever since.

Now, St. Clair officials have new information about why the road is sliding toward the river.

Geologists hired in July blame the problem on soupy clay deep beneath the road, a problem that will require deep pockets to resolve.

“We’ve got to meet, talk about this, get some numbers, find out timeframes, cost and all that,” said Mayor Bill Cedar. “Ideally? In my dream world, they give us a price, and (Superintendent Mike) Booth finds some sort of grant for it, and it lasts another 50 years.”

St. Clair City Council members got the results from G2 Consulting of Troy at their regular meeting last week.

G2's Noel Hargrave-Thomas said top 13 feet under the highway was found to be “very competent soil.” But the soils below 20 feet were soft, “toothpaste-type clays.”

That ooze is to blame for the sinking highway, he said.

The protective steel sheeting installed five years ago was doing its job, Hargrave-Thomas said, but because it buttressed a layer of soil that wasn’t to blame, the “sensitive clays” below the wall line remained free to move. They were encouraged to move, he said, because the roadway reconstruction “remolded” the slope that layer, tilting the ground toward the river.

“Essentially, the earth wants to level itself,” Hargrave-Thomas said.

The two options, he said, would be to “fill at the toe or remove the load on top.” Removing the load on top would mean lowering the roadway, but that isn’t possible because of the nearby railroad grade.

“Since we can’t do that, we can replace it with what we call lightweight fill, which is essentially Styrofoam blocks,” he said. “So we would come in there, excavate out 5 to 6 feet, we would place the Styrofoam blocks in there and reconstruct the road over it.”

Stephen Pangori, of engineering firm Anderson, Eckstein and Westrick, said the work could cost between $250,000 and $300,000 for the blocks, and $500,000 and $525,000 including road reconstruction.

Cedar asked if moving the road east, away from the river, was possible. Pangori said it was, but there were no numbers or “detained studies on realigning the road.”

“I can tell you, though, there’s a lot of complications with that,” he said. The lightweight fill is “probably the most economical” option, said.

Councilman Tom McCartney said doing nothing wasn’t option.

Mike LaPorte questioned G2's confidence in the study.

“If we proceed with the Styrofoam (blocks), what warranty would be there?” LaPorte asked. “Because our last project, we sort of got stung.”

Booth has said the city will need to utilize the data from G2, as well as results of seismic studies on the highway done by a Traverse City oil producer who was surveying for oil or natural gas under the city.

“The ultimate goal is to reopen St. Clair Highway,” he said in an email Dec. 7.

Though “some numbers were thrown out,” Cedar said taking time to look over G2’s results is just the first step.

It was uncertain when at future council meetings the highway would reenter council’s conversation.

Contact Jackie Smith at (810) 989-6270 or jssmith@gannett.com. Follow her on Twitter @Jackie20Smith.